Built the box and something sounds wrong? Match the symptom below. Every row is sourced and cross-validated in this site's enclosure-design research notes — this page is the quick reference distilled from that sourced material.
| Symptom | Likely cause | Fix |
|---|---|---|
| Buzzing/rattling from the box itself | Unbraced panel flex, or a loose T-nut/screw | Add bracing; retighten or replace T-nuts |
| Hissing/whooshing from the port at high volume ("chuffing") | Port area too small for the SPL being demanded (below the 12–16 in²/ft³ floor, or port diameter under ~40% of driver diameter), or sharp unflared port edges | Increase port area/diameter (re-solve length to hold tuning — use the Port Calculator); round over or flare the port opening's edges; as a last resort, turn gain down |
| Faint hissing around the driver itself at high SPL | Air leaking past the driver gasket or a seam | Re-seat/replace the driver gasket; recheck glue seams with a smoke test or soapy water test under pressure |
| Driver visibly over-excursing / bottoming out | Box tuned too high for the power applied, box undersized for the driver, or (for shared-chamber dual builds) one driver failed and the survivor inherited double the airspace | Verify box volume against the driver's actual T/S parameters; check both drivers if shared chamber; reduce gain |
| Sound is "loose"/"flabby," bass seems unfocused | Box too large for the driver's parameters | Recompute sealed volume via Small's formula, or reduce net volume (bracing/fill) |
| Sound is "tight"/thin, lacks low-end extension | Box too small for the driver's parameters, OR (for sealed boxes only) legitimately fixable with polyfill if the shortfall is modest | See the polyfill note below; otherwise the box needs to be rebuilt larger |
| Sub goes completely silent (or feels "dead") below a certain volume or bass note, but the amp doesn't shut off | Amp's subsonic filter is set too high or absent, letting the driver try to reproduce frequencies below the box's tuning — unloading below tuning is cited as the single most common cause of ported subwoofer failure | Set the amp's subsonic filter about 10 Hz below the box's actual tuning frequency (see the Port Calculator tuning result); never run a ported box with the subsonic filter off or set below tuning |
| Distorted/crackling sound, or a burning smell from the amp or sub | Amplifier clipping (gain set too high for the input signal, forcing a distorted square-wave-like output) or an already-damaged/burnt voice coil | Set gain properly with an oscilloscope or a known clean test tone/DMM method, not by ear at max volume; if the smell/distortion persists at reasonable gain, the coil may already be damaged and the driver needs inspection or replacement |
| Amp seems to run out of power / doesn't sound as loud as its rating suggests, especially near the tuning frequency | Box impedance/voltage rise near the tuning frequency — a normal electrical property of ported and sealed boxes that makes the amp deliver less current and power right around resonance than its rated spec implies | This is expected behavior, not a fault — it's the amp's actual delivered power at that specific frequency, not a wiring or amp problem; verify wiring/final impedance is correct before assuming something is broken |
| Rattling that seems to come from the vehicle itself, not the box | License plate, interior trim panels, loose cargo/tools, or anything else in the vehicle resonating at frequencies your new system now produces that the factory system never reached | Isolate by playing a slow sine sweep and pausing at the rattling frequency — tighten the license plate, add foam/felt behind loose trim panels, and secure or remove loose cargo; this is extremely common after a first subwoofer install and is not a box or driver problem |
Loosely filling roughly half a sealed box's internal volume with polyester fiber fill (~1 lb/ft³) makes the trapped air behave more isothermally, raising the apparent internal volume by roughly 10–25% (small boxes at the high end). This is a genuine, physically real effect — but it's a fine-tuning tool, not a rescue for a grossly undersized box. It will not "double" a box's effective volume and does nothing useful in a box already at or above the driver's target volume.